This invention relates to fuel cells and, more particularly, to a fuel cell employing a substantially immobilized electrolyte imbedded therein and having a laminated matrix assembly disposed between the electrodes of the cell for holding and distributing the electrolyte. As defined herein, a substantially immobilized electrolyte is an electrolyte contained within the matrix assembly in such a manner that permits the migration of the electrolyte within the layers of the matrix while at the same time prohibiting the free flow of the electrolyte out of the matrix. The matrix comprises at least two layers of material wherein at least one of the lamina comprises a non-conducting fibrous material such as silicon carbide whiskers and wherein the individual lamina have differing void morphologies for distributing electrolyte into the region between the electrodes, the electrolyte being used in connection with the generation of electricity.
Much research is being done in the area of fuel cell technology in order to provide ever increasing amounts of electric power and for operating such cells over longer periods of time without any need for shutdown to accomplish maintenance. As compared to other methods of generation of electric power from combustible fuels, a fuel cell has a higher efficiency and is also characterized by a simplicity of physical structure in that such cells can be constructed without any moving parts.
While a variety of electrochemical reactions are known for the conversion of fuel into electricity, one well-known form of cell utilizes the reaction between oxygen and hydrogen, the hydrogen serving as the fuel. One common form of construction for the hydrogen-oxygen cell is the laminated structure wherein the electrodes are spaced apart by a porous layer of material which holds an electrolyte, the porous layer being of such a structure that the electrolyte becomes a quasi solid. Concentrated phosphoric acid is an example of a suitable electrolyte. The hydrogen is guided by passageways behind the active region of the anode and the oxygen is guided by passageways behind the active region of the cathode. At the anode, the hydrogen gas dissociates into hydrogen ions plus electrons in the presence of a catalyst, typically a precious metal such as platinum or platinum with other metals. The hydrogen ions migrate through the electrolyte to the cathode in a process comprising ionic current transport while the electrons travel through an external circuit to the cathode. In the presence of a catalyst at the cathode, the hydrogen ions, the electrons, and molecules of oxygen combine to produce water.
In order to provide for the physical placement of the respective reactants at the catalyst layers of the anode and cathode, layers of materials having hydrophilic and hydrophobic properties are disposed in an arrangement contiguous to the catalyst layers. They permit the electrolyte and the oxygen at the cathode and hydrogen at the anode to contact the catalyst layer. The hydrophobic material is provided with pores of sufficiently large size to permit the gaseous hydrogen and the gaseous oxygen to freely flow through the material so as to come into contact with the catalyst.
Details of the construction of fuel cells, and of the component parts thereof, are disclosed in U.S. Pat. Nos. 3,453,149 of Adlhart and 4,064,322 of Bushnell. These two patents show structures for guiding the gaseous reactants into the regions of the catalyst. In addition, the Bushnell patent discloses a space within a cell for the storage of electrolyte so as to compensate for any changes in the quantity of electrolyte available for ion transport. An assembly which combines together a plurality of fuel cells into a single power source is disclosed in U.S. Pat. No. 4,175,165 of Adlhart. This patent also discloses a manifold for the simultaneous feeding of the reactant gases to the cathode and anode of the respective cells. The foregoing three patents are incorporated herein by reference in their entirety.
A problem arises during the operation of a fuel cell in that the cell has electrolyte losses. For instance, as a result of electrolyte volume changes, such as those due to temperature and composition changes, electrolyte can be driven out of the matrix and be permanently lost for use within the matrix. A fuel cell with essentially immobilized electrolyte has limited capacity for the storage of reserve electrolyte therein. Thus, depending on the amount of such storage capacity, there is a limitation on the length of time during which the fuel cell can be operated before shutdown for maintenance. Such maintenance includes the replenishment of the amount of electrolyte in the requisite concentration.
A related problem is found in the distribution of electrolyte in the porous layer between the electrodes. The electrolyte is normally distributed uniformly throughout the porous layer at the time of the construction of the cell. However, later, during operation of the cell, the distribution of the electrolyte can become less uniform. For example, there may be greater losses at the edges of the cell than at the central portion thereof. Even though the porous layer is initially charged completely with electrolyte, there would not be sufficient electrolyte held by the matrix to allow adequate compensation for the selective diminution of electrolyte at various sites along the electrodes and along the layer. In those areas wherein the electrolyte disappears completely, there could even result a space through which the oxygen and the hydrogen can mix with consequential damage to the cell.
An attempted solution of the foregoing problem by the use of larger or smaller pores in such porous layer is of little help in solving this problem. Enlargement of the pore size reduces the capillary forces and, hence, the effectiveness of the layer as a barrier to the mixing of the gaseous reactants. Reduction of the pore size reduces the liquid transport rate and, hence, diminishes the possibility of maintaining uniform distribution of the electrolyte.
Additional problems arise from the complexity of the structure required to lead the electrolyte in from a region of storage to the region of electrochemical activity alongside the layers of the catalyst. Such electrolyte lead-in structures are described in the foregoing Bushnell patent. In particular, it is noted that such structures tend to increase the size of the cell, to increase resistance losses associated with the flow of electronic current, and to decrease the surface area available for the electrochemical reactions.
Many of the foregoing problems were solved as disclosed in U.S. Pat. No. 4,467,019, and pending U.S. patent application Ser. No. 568,619. The problems were solved by constructing a fuel cell of a laminated structure wherein the electrodes are spaced apart by a matrix assembly with layers of varying porosities. As disclosed therein, the matrix assembly is composed of a central layer having relatively large pores comprising a conducting carbon fiber material. This layer is sandwiched between two outer non-conducting layers having relatively small pores.
While such construction solves many of the problems discussed above, it too has its problems. Since the central layer of the matrix is composed of a conducting material, it is preferably sandwiched by at least two non-conducting layers to minimize the possibility of electronic short circuits that arise from stray fibers penetrating through the outer layers to the surface of the electrode. Because of the overall thickness of the three layer matrix, there are relatively large internal resistance losses, which result in a decreased cell performance.